In handling digital data, various security measures such as data encryption are utilized for preventing unauthorized accesses. Security measures for image data include techniques to embed a visible watermark in image data for displaying an image irrelevant to original contents over a reproduced image so as to prevent a user without valid authority from reproducing and utilizing the contents.
Visible watermarking is implemented by applying techniques for embedding a digital watermark in an image. Specifically, using a technique for embedding invisible watermark information (such as fingerprinting) in an image for the purpose of copyright protection, tracking of unauthorized use, and detection of tampered data, a visible watermark pattern is embedded instead of invisible watermark information or in addition to invisible watermark information (see Patent Document 1, for example). This operation causes the visible watermark pattern to appear over the original motion picture when the video contents into which the visible watermark has been embedded is replayed, thus obstructing one's view. Therefore, by giving information for eliminating the visible watermark to authorized users, such a system can be constructed that allows only users having valid authority can replay and see the original motion picture.
In the case image data is encrypted, an image cannot be reproduced at all if the cipher cannot be decrypted, whereas masking of an image with a visible watermark can implement various aspects depending on purposes such as covering an entire screen to prevent one to recognize a reproduced image at all and covering a portion of the screen to allow one to guess what is shown on the screen, providing greater utility.
FIG. 15 shows examples of a screen that shows a content image and one that shows the same image together with an image showing a visible watermark covering some portions of the image.
There has been a completely lossless method for embedding and removing a visible watermark in/from image data for still picture data in JPEG format and the like (see Patent Document 1, for example). However, for motion picture data that is produced by digitalizing video contents, no effective technique for embedding visible watermarks had been proposed.
Motion picture data such as video is typically handled (preserved, processed, or transferred) with the information compressed because such data involves a vast amount of data. In MPEG format, which is used for DVD video and the like, compression for motion picture data is performed through encoding using DCT (Discrete Cosine Transform) and motion-compensated inter-frame predictive coding (see Non-Patent Document 1, for example).
[Patent Document 1]
Japanese Patent No. 3269015
[Non-Patent Document 1]
“Illustrated Tutorial of the latest MPEG” editorial supervision by Hiroshi Fujiwara, Multimedia Communication Workshop ed. ASCII, Jul. 26, 1994